Two becomes one

According to Richard Feynman, there is one experiment that exposes "the only mystery" of quantum mechanics. Light from a single source is split into two beams that travel along different paths. Where the beams recombine, two detectors measure how the two waves interfere with each other: detector S fires if the beams interfere constructively and detector D fires if they interfere destructively, cancelling each other out. For a beam with no interference, either S or D will fire, each with a 50-50 probability. When the two paths have the same length, the waves will be in phase where they recombine and the beams will interfere constructively, so detector S keeps firing and D never fires.

Now suppose the light is so feeble that photons can only travel through the apparatus one at a time. That interference remains. The startling implication is that each photon has to travel along both paths simultaneously. The same goes for beams of electrons, neutrons, atoms, or molecules. Zeilinger has even seen this happening to buckyballs--big, football-shaped carbon molecules.

But if instruments are installed to measure which path the photons travel down, the detectors start firing randomly: interference is destroyed. How, except by magic, can this be reconciled with the previous experiment on the same apparatus? How do photons know whether to go down only one path, or both?

Zeilinger's answer is that our choice of measurement is putting that information into the photon. But it can only carry one bit. So if we arrange the experiment so that the photon is destined to trigger detector S, that bit is used up, and we can have no knowledge of which path it traversed. On the other hand, if we decide to know which path it travelled, we cannot predict whether S or D will fire.

This experiment highlights another troubling aspect of quantum mechanics, called the measurement problem. Each photon seems to undergo a mysterious metamorphosis from a quantum wave to a classical particle in the act of measurement. But according to Zeilinger's principle, we simply cannot know enough about the photon to call it either wave or particle. Zeilinger's elementary system is no more than a carrier of information.